Typical swather tractors include a frame which carries a cab and engine with a transverse frame assembly which carries on each of a pair of depending frame leg a respective one of a pair of driven wheels. The transverse frame assembly also carries a pair of lift arms each for supporting a header of the tractor to be moved in front of the tractor for crop harvesting.
At the rear of the frame is a transverse rear axle assembly which carries a pair of rear wheels. The rear wheels are castor wheels with a vertical castor shaft pivotal around a vertical axis at the respective end of the axle assembly and a single wheel mounted on a hub offset from the vertical axis so that the wheel hub defines a horizontal rotation axis for the wheel which is located at a position trailing the vertical castor axis.
The castor wheel is allowed to rotate freely and the steering for the tractor is provided by a differential in drive speed between the driven wheels so that the castor wheels merely follow the movement generated by the drive wheels and the frame.
In application filed Apr. 28, 2005, Ser. No. 11/116,418 published as 2006/0201732 (now U.S. Pat. No. 7,159,687 issued Jan. 9, 2007) and Canadian application filed Apr. 27, 2005, Serial No: 2,513,614 and assigned to the present assignee is shown a machine in which the machine travels in transport mode with the castor wheels leading the drive wheels (engine forward), and in this position the machine is much more stable and higher speeds are possible.
Thus the swather tractor can be driven in a cab forward position in a working mode with a header on the forward end. It is rotated to engine forward in the transport position for more stable higher speed travel. The driver's console is rotated in the cab with the steering and speed control elements moved with the seat for the driver to face forwards and its position is detected by switches. In the engine forward position for transportation, the control system detects the seat position and operates the controls for higher speed drive and to prevent operation of the header. It is known that the tractor that is transported in the normal direction (header first) is less stable than when the engine is first. Thus the castor wheels must be freely turnable to follow the direction of travel and must be free to rotate fully around the vertical castor axis.
To vary the speed of the machine, both pump displacements are changed simultaneously. To steer the machine, the displacement of each pump is varied relative to each other so that the flow to each wheel is different which causes the machine to turn.
In application filed Jul. 25, 2005, Ser. No. 11/188,293 published as 2007/0017712 (now issued as U.S. Pat. No. 7,306,062 issued Dec. 11, 2007) and Canadian application filed Jul. 21, 2005, Serial No: 2,505,458 and assigned to the present assignee is shown a steering and speed control arrangement for the above machine.
The disclosures of the above patents are incorporated herein by reference or may be referred to for further details.
The center of gravity of the machine is somewhere between the drive wheels and the caster wheels. Any acceleration or deceleration of the machine imparts a force at the center of gravity. When in a right turn, there is a force on the machine at the center or gravity. This force increases with speed and smaller turning radius. The direction of this force is causing the machine to want to go into an even sharper turn and the operator has to fight this constantly with every movement of the rear of the machine, making the machine hard to control. This is why the speed of windrowers has been limited to approximately 16 mph up to now.
It has long been known that a windrower, which is hydrostatically steered while traveling with the caster wheels trailing the machine, can become unstable at speeds above about 15 mph. Also, when a hydrostatically steered machine is stopped or slowed abruptly, the steering/drive system may exceed relief pressure and steering control is then lost. In addition, the automatic steering of these machines in the field becomes difficult at higher speeds due to the imprecise nature of the hydrostatic steering system.
Dual path traction propelled machinery such as the windrower requires a differential moment to be applied by the traction wheels simultaneously to the propulsion forces in order to generate a steering function. Ample differential torque can be generated when the hydraulic transmission is in a field range that provides high torque/low speed. In medium field range and transport range that provides lower torque/higher speed, steering commands are less effective and when maximum torque is required for braking operations, available steering torque is compromised. A windrower feature is the infinitely variable steering during field operation by utilizing a pair of caster wheels to support one end of the machine. However during transport and high speed field operations, full caster function is not required.